Up to the present time, there have been many proposals of inventions which measure the wall thickness of an object of interest using a gamma ray wall thickness gauge and set and correct the rolling conditions based on the measured results (see Patent Document 1, for example).
A gamma ray wall thickness gauge measures wall thickness based on the amount of attenuation of gamma rays passing through an object of interest. On account of this principle of measurement, it is not possible to measure the wall thickness of a hollow shell into which a mandrel bar has been inserted using a gamma ray wall thickness gauge. Therefore, it is of course not possible to measure the wall thickness between the stands of a mandrel mill with a gamma ray wall thickness gauge. Even on the exit side of a mandrel mill, thickness measurement can only be performed with a retractable mandrel mill in which the inserted mandrel bar is retracted towards the entrance side after the completion of elongation rolling. Moreover, even with a retractable mandrel mill, the wall thickness can be measured only in a location spaced by a certain amount from the immediate vicinity of the exit of the mill. Due to these limitations, there is naturally a limit to the extent to which the results of wall thickness measurement by a gamma ray thickness gauge can be utilized to carry out high accuracy control.
Thus, in a control method for a mandrel mill using a conventional gamma ray wall thickness gauge, there is the fundamental problem that it is not possible to measure the wall thickness of a hollow shell into which a mandrel bar is inserted. Due to this problem, the following four specific problems exist.